Despite being essentially water-free, nominally anhydrous minerals such as plagioclase and pyroxene represent the biggest reservoir of water in most lunar rocks due to their sheer abundance. Apatite, which incorporates F, Cl, and OH into its mineral structure as essential crystal components, on the other hand, is the only other volatile-bearing phase common in lunar samples. Here, we present the first coordinated study of volatiles (e.g., H2O, Cl, F, and S) in nominally anhydrous minerals combined with isotopic measurements in apatite from the ancient lunar basalt fragments from meteorite Miller Range (MIL) 13317. Apatite in MIL 13317 basalt contains similar to 2000 ppm H2O and has an elevated SD values (+ 523-737 parts per thousand), similar to Apollo mare basalts, but has high delta Cl-37 values (+ 29-36 parts per thousand), similar to apatite found in several KREEP-rich samples. MIL 13317 is unique compared with other lunar basalts; it has both elevated SD and delta Cl-37 values currently only observed in highlands sample 79215 (a granulitic impactite). Based on measurements of H2O in nominally anhydrous minerals and in apatite, the source magma of MIL 13317 basalt is estimated to contain similar to 130-330 ppm H2O. Assuming reasonable levels of partial melting of the lunar mantle and magmatic degassing during eruption of the basalt, the Moon contained at least one reservoir with < 100 ppm H2O, a delta D value of < 0 parts per thousand similar to carbonaceous chondrites, and extensively fractionated Cl isotopes prior to 4.332 Gyr, the crystallization age of the MIL 13317 basalt.

We compare the stable isotope compositions of Zn, S, and Cl for Apollo mare basalts to better constrain the sources and timescales of lunar volatile loss. Mare basalts have broadly elevated yet limited ranges in delta Zn-66, delta S-34, and delta Cl-37(SBC+WSC) values of 1.27 +/- 0.71, 0.55 +/- 0.18, and 4.1 +/- 4.0 parts per thousand, respectively, compared to the silicate Earth at 0.15, -1.28, and 0 parts per thousand, respectively. We find that the Zn, S, and Cl isotope compositions are similar between the low- and high-Ti mare basalts, providing evidence of a geochemical signature in the mare basalt source region that is inherited from lunar formation and magma ocean crystallization. The uniformity of these compositions implies mixing following mantle overturn, as well as minimal changes associated with subsequent mare magmatism. Degassing of mare magmas and lavas did not contribute to the large variations in Zn, S, and Cl isotope compositions found in some lunar materials (i.e., 15 parts per thousand in delta Zn-66, 60 parts per thousand in delta S-34, and 30 parts per thousand in delta Cl-37). This reflects magma sources that experienced minimal volatile loss due to high confining pressures that generally exceeded their equilibrium saturation pressures. Alternatively, these data indicate effective isotopic fractionation factors were near unity. Our observations of S isotope compositions in mare basalts contrast to those for picritic glasses (Saal and Hauri 2021), which vary widely in S isotope compositions from -14.0 to 1.3 parts per thousand, explained by extensive degassing of picritic magmas under high-P/P-Sat values (>0.9) during pyroclastic eruptions. The difference in the isotope compositions of picritic glass beads and mare basalts may result from differences in effusive (mare) and explosive (picritic) eruption styles, wherein the high-gas contents necessary for magma fragmentation would result in large effective isotopic fractionation factors during degassing of picritic magmas. Additionally, in highly vesiculated basalts, the delta S-34 and delta Cl-37 values of apatite grains are higher and more variable than the corresponding bulk-rock values. The large isotopic range in the vesiculated samples is explained by late-stage low-pressure vacuum degassing (P/P-Sat similar to 0) of mare lavas wherein vesicle formation and apatite crystallization took place post-eruption. Bulk-rock mare basalts were seemingly unaffected by vacuum degassing. Degassing of mare lavas only became important in the final stages of crystallization recorded in apatite-potentially facilitated by cracks/fractures in the crystallizing flow. We conclude that samples with wide-ranging volatile element isotope compositions are likely explained by localized processes, which do not represent the bulk Moon.

We conducted a petrologic study of apatite within eight unbrecciated, non-cumulate eucrites and two monomict, non-cumulate eucrites. These data were combined with previously published data to quantify the abundances of F, Cl, and H2O in the bulk silicate portion of asteroid 4 Vesta (BSV). Using a combination of apatite-based melt hygrometry/chlorometry and appropriately paired volatile/refractory element ratios, we determined that BSV has 3.0-7.2 ppm F, 0.39-1.8 ppm Cl, and 3.6-22 ppm H2O. The abundances of F and H2O are depleted in BSV relative to CI chondrites to a similar degree as F and H2O in the bulk silicate portion of the Moon. This degree of volatile depletion in BSV is similar to what has been determined previously for many moderately volatile elements in 4 Vesta (e.g., Na, K, Zn, Rb, Cs, and Pb). In contrast, Cl is depleted in 4 Vesta by a greater degree than what is recorded in samples from Earth or the Moon. Based on the Clisotopic compositions of eucrites and the bulk rock Cl-/F ratios determined in this study, the eucrites likely formed through serial magmatism of a mantle with heterogeneous delta Cl-37 and Cl/F, not as extracts from a partially crystallized global magma ocean. Furthermore, the volatile depletion and Cl-isotopic heterogeneity recorded in eucrites is likely inherited, at least in part, from the precursor materials that accreted to form 4 Vesta and is unlikely to have resulted solely from degassing of a global magma ocean, magmatic degassing of eucrite melts, and/or volatile loss during thermal metamorphism. Although our results can be reconciled with the past presence of wide-scale melting on 4 Vesta (i.e., a partial magma ocean), any future models for eucrite petrogenesis involving a global magma ocean would need to account for the preservation of a heterogeneous eucrite source with respect to Cl/F ratios and Cl isotopes. Published by Elsevier Ltd.

The traditional view of a dry, volatile-poor Moon has been challenged by the identification of water and other volatiles in lunar samples, but the volatile budget delivery time (s), source (s) and temporal evolution remain poorly constrained. Here we show that hydrogen and chlorine isotopic ratios in lunar apatite changed significantly during the Late Accretion (LA, 4.1-3.8 billion years ago). During this period, deuterium/hydrogen ratios in the Moon changed from initial carbonaceous-chondrite-like values to values consistent with an influx of ordinary-chondrite-like material and pre-LA elevated delta Cl-37 values drop towards lower chondrite-like values. Inferred pre-LA lunar interior water contents are significantly lower than pristine values suggesting degassing, followed by an increase during the LA. These trends are consistent with dynamic models of solar-system evolution, suggesting that the Moon's (and Earth's) initial volatiles were replenished similar to 0.5 Ga after their formation, with their final budgets reflecting a mixture of sources and delivery times.

Compared to most other planetary materials in the Solar System, some lunar rocks display high delta Cl-37 signatures. Loss of Cl in a H << Cl environment has been invoked to explain the heavy signatures observed in lunar samples, either during volcanic eruptions onto the lunar surface or during large scale degassing of the lunar magma ocean. To explore the conditions under which Cl isotope fractionation occurred in lunar basaltic melts, five Apollo 14 crystalline samples were selected (14053,19, 14072,13, 14073,9, 14310,171 along with basaltic clast 14321,1482) for in situ analysis of Cl isotopes using secondary ion mass spectrometry. Cl isotopes were measured within the mineral apatite, with delta Cl-37 values ranging from +14.6 +/- 1.6 parts per thousand to +40.0 +/- 2.9 parts per thousand. These values expand the range previously reported for apatite in lunar rocks, and include some of the heaviest Cl isotope compositions measured in lunar samples to date. The data here do not display a trend between increasing rare earth elements contents and delta Cl-37 values, reported in previous studies. Other processes that can explain the wide inter- and intra-sample variability of delta Cl-37 values are explored. Magmatic degassing is suggested to have potentially played a role in fractionating Cl isotope in these samples. Degassing alone, however, could not create the wide variability in isotopic signatures. Our favored hypothesis, to explain small scale heterogeneity, is late-stage interaction with a volatile-rich gas phase, originating from devolatilization of lunar surface regolith rocks similar to 4 billion years ago. This period coincides with vapor-induced metasomastism recorded in other lunar samples collected at the Apollo 16 and 17 landing sites, pointing to the possibility of widespread volatile-induced metasomatism on the lunar nearside at that time, potentially attributed to the Imbrium formation event. (C) 2018 Elsevier Ltd. All rights reserved.

Experimental degassing of H-, F-, Cl-, C-, and S-bearing species from volatile-bearing magma of lunar composition at low pressure and fo(2), close to the quartz-iron-fayalite buffer (QIF) indicates that the composition of the fluid/vapor phase that is lost changes overtime. A highly H-rich vapor phase is exsolved within the first 10 min of degassing leaving behind a melt that is effectively dehydrated. Some Cl, F, and S is also lost during this time, presumably as HCI, HF, and H2S gaseous species; however much of the original inventory of Cl, F, and S components are retained in the melt. After 10 min, the exsolved vapor is dry and dominated by S- and halogen-bearing phases, presumably consisting of metal halides and sulfides, which evolves over time toward F enrichment. This vapor evolution provides important constraints on the geochemistry of volatile-bearing lunar phases that form subsequent to or during degassing. The rapidity of H loss suggests that little if any OH-bearing apatite will crystallize from surface or near surface (similar to 7m) melts and that degassing of lunar magmas will cause the compositions of apatites to evolve first toward the F-CI apatite binary and eventually toward end-member fluorapatite during crystallization. During the stage of loss of primarily H component from the melt, Cl would have been lost primarily as HCI, which is reported not to fractionate Cl isotopes at magmatic temperatures (Sharp et al. 2010). After the loss of H-bearing species, continued loss of CI would result in the degassing of metal chlorides, which have been proposed as a mechanism to fractionate Cl isotopes (Sharp et al. 2010). After the onset of metal chloride degassing, the delta Cl-37 of the melt would necessarily increase to +6 (82% Cl loss), +8 (85% Cl loss), and +20 parts per thousand (95% Cl loss) at 1, 4, and 6 h, respectively, which was approximated using a computed trajectory of delta Cl-37 values in basalt during degassing of FeCl2. This strong enrichment of Cl-37 in the melt after metal chloride volatilization is fully consistent with values measured for the non-leachates of a variety of lunar samples and would be reflected in apatites crystallized from a degassing melt. Our results suggest that a range in delta Cl-37 from 0 to >20 parts per thousand is expected in lunar apatite, with heavy enrichment being the norm. While 95% loss in the initial Cl content of the melt (280 ppm Cl left in the melt) would cause an increase to +20 parts per thousand in delta Cl-37, the ability to measure this increase in a lunar sample is ultimately dependent upon the starting Cl abundances and whether or not a mechanism exists to concentrate the remaining CI such that it can be subsequently analyzed with sufficient accuracy. Therefore, the higher the starting Cl abundances in the initial melts, the heavier delta Cl-37 values that can be measurably preserved. Importantly, such enrichments can occur in spite of high initial hydrogen contents, and therefore, our experiments demonstrate that elevated values of delta Cl-37 cannot be used as supporting evidence for an anhydrous Moon. Furthermore, if the H-bearing vapor has a significant H-2 component, this process should also result in strong enrichment of delta D in the residual magmas that reach the lunar surface or near-surface environment. Apatites within some mare basalts exhibit elevated delta D of 1000 parts per thousand depending on the initial value (Tartese and Anand 2013) in addition to the delta Cl-37 values, but elevated delta Cl-37 values are accompanied by only modest enrichments in 51) in apatites from samples of the highlands crust (McCubbin et al. 2015a).